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An expected aggregate net thickness of coals in the Carbondale and Spoon formations in the Illinois Basin is 15 to 45 feet except in areas of outcrop or subcrop. The BTU content of the coal varies from 11,000 to 15,500 DMMF. Moisture content varies from 5% to 19%; ash content varies from 1% to 25%; and volatile matter content varies from 28% to 41%.
More detailed information for the No. 6 (Herrin) seam from: https://www.ecn.nl/phyllis/
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BTW, 1 kJ/kg = 0.4299 Btu/ lbm